The present invention relates to a method for controlling the characteristics of an aqueous solution, and more particularly to a method for controlling the characteristics of a liquid mixture used in the surface treatment of a web material such as an aluminum web, a cold-rolled steel web, etc.
A control system combining feedforward and feedback controls is described in a report entitled "A New Combination of Feedforward and Feedback Controls" in the briefing text delivered at the 22nd SICE Science Lecture Meeting, 1983.
This system is shown in FIG. 3 of the accompanying drawings, and comprises a flow meter 11 to meter the rate of fluid flow into a heat exchanger 10 and to provide an appropriate electric signal indicating the detected rate of flow (which is hereinafter referred to as a flow rate signal (FRS)). The rate of flow of vapor or steam at a certain temperature which heats the fluid in the heat exchanger 10 is regulated by a control valve 12. A steam flow control unit 13 changes the opening of the control valve 12 so as to regulate the rate of flow of the steam. A thermometer 14 detects the temperature of the heated fluid discharged from the heat exchanger 10 to provide an appropriate electric signal (which is hereinafter referred to as a temperature signal (TS)). The temperature of the heated fluid detected by the thermometer 14 is compared with a reference temperature previously set by a temperature setting unit 9 in a comparison circuit 15. A feedforward operator 16 calculates an appropriate rate of steam flow based on the result of the comparison of the temperatures in the comparator circuit 15 so as to provide the steam flow controlling unit 13 with a signal either to increase the rate of flow of the steam when the detected temperature is lower than the reference temperature or to decrease it when the detected temperature is higher than the reference temperature. Because the change of the rate of flow of the steam depends upon the change of the amount of heat which the heated fluid has received, a closed loop is formed for feedback control. The feedforward operator 16 receives the flow rate signal FRS from the flow meter 11 and performs an analog calculation based on the flow rate of the heated fluid so as to provide the steam flow control unit 13 with a signal either to increase the flow rate of the steam when the detected flow rate of the heated fluid is low or to decrease it when the detected flow rate of the heated fluid is high.
In the above-described control system, because the feedforward operator 16 performs an analog operation and data for the analog operation is measured before the analog operation is performed, it is hard to perform real-time operations. On the other hand, in a treating apparatus for treating uniformly the whole length of a continuously moving web in the direction in which it is driven, it is unavoidable that the treating vessel or tank containing the liquid mixture or aqueous solution for treating the surfaces of the web will be elongated in the moving direction. This leads to a relatively long delay of time in controlling the treating solution. Therefore, there will necessarily be a lack of stability in controlling the characteristics of aqueous solutions, when a solution is not controlled in real-time based on measured data and there results a relatively long time delay in controlling the treatment solution.